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The “finger-like” parapostzygapophyseal processes of Qijianglong

January 29, 2015

There’s a new mamenchisaurid in town! It’s called Qijianglong (“dragon of Qijiang”), and it’s the work of Xing et al. (2015).

Life restoration of Qijianglong, by Cheung Chungtat.

As far as I can make out, the life restoration is also due to Xing Lida: at least, every instance of the picture I’ve seen says “Credit: Xing Lida”. If that’s right, it’s an amazing display of dual expertise to produce both the science and the art! We could quibble with details, but it’s a hundred times better than I could ever do. [Update: no, it’s by Cheung Chungtat, but being uniformly mis-attributed in the media. Thanks to Kevin for the correction in the comment below.]

There’s a mounted skeleton of this new beast in the museum local to where it was found, though I don’t know how much of the material is real, or cast from the real material. Here it is:

A reconstructed skeleton of Qijianglong now on display in Qijiang Museum

A new sauropod is always great news, of course, and it’s a source of shame to us that we cover so few of them here on SV-POW!. (Just think of some of the ones we’ve missed recently … Leikupal, for example.)

But as is so often the case, the most interesting thing about this new member of the club is its vertebrae — specifically the cervicals. Here they are:

(At first, I couldn’t figure out what this pocdl abbreviation meant. Then I realised it was a vanilla posterior centrodiapophyseal lamina. Come on, folks. That element has had a standard abbreviation since 1999. Let’s use our standards!)

The hot news in these cervicals is the presence of what the authors call “a distinct finger-like process extending from the postzygapophyseal process beside a zygapophyseal contact”. They don’t give a name to these things, but I’m going to call them parapostzygapophyses since they’re next to the postzygapophyses. [Update: see the comment from Matt below.]

You can get some sense of this morphology from the figure above — although it doesn’t help that we’re looking at tiny greyscale images which really don’t convey 3d structure at all. The best illustration is part J of the figure:

What are these things? The paper itself says disappointingly little about them. I quote from page 9:

From the axis to at least the 14th cervical vertebra, a finger- like process extends posteriorly above the postzygapophysis and overlaps onto the dorsolateral surface of the prezygapophysis of the next vertebra (Fig. 11I, J). These processes are unique to Qijianglong, unlike all previously known mamenchisaurids that are preserved with cervical vertebrae (e.g., Chuanjiesaurus, Mamenchisaurus spp., Omeisaurus spp., Tonganosaurus). Therefore, the neck of Qijianglong presumably had a range of motion restricted in sideways.

That’s it.

So what are these things? The authors — who after all have seen the actual fossils, not just the rather inadequate pictures — seem to assume that they are a stiffening adaptation, but don’t discuss their reasoning. My guess — and it’s only a guess — it that they assumed that this is what was going on with these processes because it’s what people have assumed about extra processes on xenarthrous vertebrae. But as best as I can determine, that’s not been demonstrated either, only assumed. Funny how these things seem to get a pass.

Armadillo lumbar vertebrae in posterior, anterior and right lateral views.

So what are these processes? It’s hard to say for sure without having seen the fossils, or at least some better multi-view photos, but the obvious guess is that they are our old friends epipophyses, in extreme form. That is, they are probably enlarged attachment points for posteriorly directed dorsal muscles, just as the cervical ribs are attachment points for posteriorly directly ventral muscles.

It’s a shame that Xing et al. didn’t discuss this (and not only because it would probably have meant citing our paper!) Their new beast seems to have some genuinely new and interesting morphology which is worthy of a bit more attention than they gave it, and whose mechanical implications could have been discussed in more detail. Until more is written about these fossils (or better photographs published) I think I am going to have to suspend judgement on the as-yet unjustified assumption that the parapostzygs were there to make the neck rigid against transverse bending.

A final thought: doesn’t JVP seem terribly old-fashioned now? It’s not just the paywall — apologies to those many of you who won’t be able to read the paper. The greyscaling of the figures is part of it — something that makes no sense at all in 2015. The small size and number of the illustrations is also a consequence of the limited page-count of a printed journal — it compares poorly with, for example, the glorious high-resolution colour multiview illustrations in Farke et al.’s (2013) hadrosaur description in PeerJ. Seems to me that, these days, all the action is over at the OA journals with infinite space — at least when it comes to descriptive papers.

Interesting beast for sure, I wonder if we are on the eve of a mamenchisaurid revolution? Maybe those strange projections have something to do with the vertical up/down foraging neck movement I heard about somewhere for these obscenely necked sauropods? Interesting the variety of mechanical approaches sauropods took all working from the same basic bauplan.

They don’t give a name to these things, but I’m going to call parapostzygapophyses since they’re next to the postzygapophyses.

Er, why? We already have a perfectly good term for backward-pointing bony spurs above the postzygs. In fact, we have three: epipophyses, dorsal tubercles, and tori dorsales (for fans of avian vertebral nomenclature). The ones in Qijianglong might not even be the longest ones known in sauropods–both Nigersaurus and Jobaria also have prominent epipophyses that strongly overhang the postzygs in at least some of their cervical vertebrae (see Fig. 6 in Wilson et al. 2011 for Nigersaurus). These almost certainly represent ossification of the proximal part of the longus colli dorsalis tendons, just as the cervical ribs are ossified longus colli ventralis tendons.

I agree that the epipophyses in Qijianglong are the most prominent ever documented in mamenchisaurids and they at least tie and maybe exceed the longest epipophyses known in other sauropods, but it’s not like they’re some bizarre new structure. At most they are one end of a spectrum of epipophyseal development in sauropods. Given that epipophyses in other sauropods have attracted a fair amount of attention in the literature already, it is surprising that the authors didn’t simply identify them as such, and cite some of those papers. We don’t have to compound that lapse by introducing a new, unnecessary term for structures whose homology is already well-understood. I dunno–maybe you proposed the term ‘parapostzygapophyses’ as a sort of placeholder, to avoid jumping to the conclusion that they are epipophyses? I would be more in favor of doing that if there was any reason at all to think that they aren’t just unusually long epipophyses.

We certainly _need_ a mamenchisaurid revolution, so I hope we’re on the eve of one. Besides the ten proposed species of Mamenchisaurus and ten proposed species of Omeisaurus, we have Chuanjiesaurus, Eomamenchisaurus, Huangshanlong, Hudiesaurus, “Moshisaurus”, Qijianglong, Tienshanosaurus, Tonganosaurus, Xinjiangtitan and Yuanmousaurus. I don’t think any species has even been compared to a majority of the others, and there’s no consistency in why some are referred to Omeisaurus or Mamenchisaurus vs. being given their own genera. Sekiya (2011) and this new Xing et al. paper have the largest cladistic analyses (using 9 and 10 species, respectively), but despite both being modifications of Wilson’s matrix, they come to completely different conclusions on phylogeny. Both do find non-Mamenchisaurus taxa nested inside that genus though, showing something’s wrong. Luckily, Xing et al. say “A thorough cladistic analysis of all mamenchisaurids should serve as a guideline for recombination and formulation of taxonomic names. This process is underway by the authors.”

Btw, Xing et al.’s definition of Mamenchisauridae as “a stem-based clade more closely related to M. constructus, M. youngi, O. junghsiensis, and O. tianfuensis than to Shunosaurus, Barapasaurus, Patagosaurus, or Spinophorosaurus” is TERRIBLE. There’s no need to include M. youngi or any Omeisaurus species as internal specifiers- if they end up closer to Neosauropoda or something, Mamenchisauridae should still be able to exist. Similarly, why exclude those latter four genera? Mamenchisauridae has priority over Barapasauridae and Shunosauridae, and the other two haven’t even had families named after them. The only potentially closely related taxon that has priority family-wise is Cetiosaurus. “All taxa more closely related to Mamenchisaurus constructus than to Cetiosaurus oxoniensis, Diplodocus longus or Camarasaurus supremus” would make FAR more sense.

Finally, if these are just epipophyses (and I agree with you they are), why come up with a new term (‘parapostzygapophyses’) for them? Surely one thing we don’t need is more overlapping sauropod vertebral nomenclature?

I’m going to call them parapostzygapophyses since they’re next to the postzygapophyses.

Er, why? We already have a perfectly good term for backward-pointing bony spurs above the postzygs.

Because I didn’t want to beg the question of their function by using a name that nails it down. I went with parapostzygs so that I could ask the question “Are the parapostzygs epipophyses?”

But assuming we all agree that they are epipophyses — and I’d really like to hear from the authors of the paper before we accept that as definitive — then, I agree, we don’t want the new term parapostzygs to get out there into the wild. So maybe it would been better not to have coined it at all.

“Because I didn’t want to beg the question of their function by using a name that nails it down.”

But since when are epipophyses defined by their function? The term just means ‘on top’ of the apophysis, in this case the postzygapophysis. In theropods, epipophyses vary from slight convexities, to longitudinal ridges, to prongs, to huge block-like protrusions. As long as it’s a bony growth over the postzygapophysis, it’s an epipophysis.

Yep, you’re right. Matt and I have been so tied up into the function of epipophyses that I’ve been suckered into thinking the name is a functional rather than physical description.

The only other thing to say about this is that Xing et al. say (in the caption to figure 11) “finger-like process lateral to postzygapophysis”; and (in the abstract) “distinct finger-like process extending from the postzygapophyseal process beside a zygapophyseal contact.” (My emphasis in both quotes.) So arguably these things are beside (“para-“) rather then above (“epi-“). But I admit that’s weak sauce.

So arguably these things are beside (“para-“) rather then above (“epi-“).

Yeah, that’s what they say, but in all of their lateral view photos, the processes are also above the postzyg facets. Which makes sense–epipophyses are often both dorsal and lateral to the postzygs. At the postero-dorso-lateral ‘corners’ of the neural arch, if you will.

Anyway, I was inspired to write a post comparing epipophyseal morphology across sauropodomorphs. Too much to do to finish it today, but I’ll try to have it up by the start of next week.

[…] at SV-POW! discuss theses weird finger like processes extending from the vertebrae of Qijianglong here. It’s an excellent article which not only discusses the weird processes in detail but also […]